The present invention relates to a braking pressure control system including a braking pressure generator, a brake-actuating device for at least one vehicle wheel, a connecting line between the braking pressure generator and the brake-actuating device, a shut-off valve in the connecting line which has two switching positions, the valve keeping the connecting line open in its basic position and closing or throttling the connecting line in its switched position, an outlet line which connects the brake-actuating device with a pressure fluid collecting means, an electromagnetically operated pressure-limiting valve in the outlet line having an adjustable opening pressure, a wheel speed sensor which senses the rotational speed of the vehicle wheel and issues a corresponding sensor signal, and an electronic controlling and evaluating unit which has an input for the sensor signal and a first output (switching output) to which a switching signal for the shut-off valve is applied, and has another output (controlling output) to which a control signal for the current-supply circuit is applied that determines the intensity of current supplied to the solenoid of the electromagnetically operated pressure-limiting valve.
German published Patent Application No. 20 21 185 discloses a brake system of this type. In the braking pressure control system mentioned in the application, a pressure-limiting valve is operated by a proportional magnet. One feature of a proportional magnet is that the force applied to the armature is proportional to the coil current irrespective of the position of the armature in the coil. When a magnet of this type is used to actuate a valve closure member, the closing force is determined by the coil current alone, irrespective of how remote the outflowing pressure medium will remove the closure member from the valve seat.
In the state of the art mentioned above, the closure member is retained on the valve seat by a rigid spring so that the opening pressure of the pressure control valve is above the maximum possible braking pressure which does not yet cause wheel lock. The operating force produced by the solenoid causes the opening pressure to decrease with rising current intensity.
The objective of a pressure control valve of this type is to replace a corresponding switching valve in the outlet line. The switching valves which are normally inserted into the outlet line have two switching positions, i.e. an open and a closed position. Pressure reduction is achieved by briefly opening the switching valve in intervals until the pressure is sufficiently decreased. It is obvious that a pressure control valve renders possible a considerably more accurate control because the desired pressure can be adjusted directly. The condition for this adjustment, however, is a precise knowledge of the control operations and the employment of an electronic evaluating unit which determines control signals for energizing the solenoid of the braking pressure control valve in consideration of sensor signals and cognizant of the control operation.
In anti-lock control systems, frequently, only the first time derivative of the rotational speed of the wheel (the wheel deceleration) is used to govern the control operation.
When the wheel deceleration exceeds a negative threshold value, anti-lock control will respond by removing pressure fluid from the wheel brake.
It has already been found that a control which takes into account the wheel deceleration does not furnish good results. Therefore, suggestions have been made to determine the slip values of the wheel and to adjust the braking pressure so that the slip of the wheel is in a range in which the adhesion coefficients are at the maximum. To determine the wheel slip, however, it is necessary to know the vehicle speed. But the vehicle speed is difficult to determine during a braking operation when the wheels of the vehicle do not roll freely. Therefore, hypothetical speeds have been introduced by way of which approximated slip values could be determined.
British Patent Application No. 2 182 740 discloses measuring the pressure in the wheel brake during a braking pressure control operation. The pressure is proportional to the brake force applied by the brake actuating device. The force which acts between tires and road surface can be determined in consideration of the wheel deceleration. The value of vehicle deceleration is obtained when the last mentioned value is added for all four wheels and divided by the vehicle mass. The speed of the vehicle, at any time of braking, may be derived from this value by a time integration. Because the wheel rotational speed is also known from the evaluation of the wheel sensor signals, the slip of the wheels can be calculated.
Therefore, an additional pressure sensor is used in the above mentioned publication.
German Patent Application No. 34 46 016 discloses a method of determining an optimal slip value. The publication also discloses determining the vehicle speed. To this end, the pressure in the master brake cylinder and the wheel rotational speed is measured. The objective is to improve and simplify the identification of the optimal brake slip by measuring the pressure in the wheel brake cylinder and the vehicle speed. A method of measurement is not indicated.